A variety of physiological processes are electrically mediated and produce associated electrical signals. For example, the sinoatrial node in a human heart generates an electrical pulse that triggers the remainder of a heartbeat in a normally functioning heart. This pulse propagates through the heart's normal conduction pathways producing electrical signals that can be observed on the surface of a patient's body. Analysis of such electrical signals has proved beneficial in evaluating the function of a patient's heart, including the detection of conditions associated with acute cardiac ischemia.
Analysis of a patient's cardiac electrical activity for detection of acute cardiac ischemic conditions is conventionally performed in a hospital setting using a 12-lead electrocardiogram (ECG) system. A conventional 12-lead ECG system measures voltage potentials sensed by ten electrodes attached to a patient and generates twelve combinations of these voltages to produce the "leads" required by the 12-lead ECG system. Of the ten electrodes in a 12-lead ECG system, four are "limb" electrodes typically placed on or near each of a patient's four limbs, and six are "precordial" electrodes positioned on the patient's chest over the heart. As an electrical impulse propagates through the heart, the ECG system repetitively measures the voltages sensed by the electrodes. Although the electrodes collectively measure the same cardiac electrical activity, the electrodes sense different voltages due to their different position on the patient with respect to the patient's heart. A time sequence of measured voltages is used to produce ECG lead data. An ECG system typically plots this data to provide graphical waveforms representing the heart's electrical activity for each lead being measured.
An example of an ECG waveform for a period of one cardiac cycle, or heartbeat, is shown in FIG. 1. The portion of a waveform representing depolarization of the atrial muscle fibers is referred to as the "P" wave. Depolarization of the ventricular muscle fibers is collectively represented by the "Q." "R," and "S" waves of the waveform. The portion of the waveform representing repolarization of the ventricular muscle fibers is known as the "T" wave. Between heartbeats, an ECG waveform returns to an "isopotential" line.
FIG. 1 also illustrates selected fiducial points labeled "q", "j", "t1", and "t2". Fiducial points define the boundaries of selected features and are used in measuring characteristics of an ECG waveform, such as the start and end of a heartbeat and the elevation of the ST portion of a heartbeat. The "q" point shown in FIG. 1 represents the start of the Q wave, the "j" point represents the end of the QRS complex, the "t1" point represents the start of the T wave, and the "t2" point represents the end of the T wave.
As noted, an analysis of a patient's ECG has proved beneficial in detecting acute cardiac ischemia in a patient. In terms of physiology, acute cardiac ischemia is a condition that arises from chronic or sudden onset of deprivation of blood, and hence oxygen, to muscles of the heart. If an ischemic condition is severe or prolonged, it can result in irreversible death or damage to myocardial cells (i.e., an infarction). A chronic cardiac ischemic condition, angina, is typically caused by narrowing of the coronary arteries due to spasms of the wall muscles or partial blockage by plaques. A sudden cardiac ischemic condition may be caused by a clot blocking the passage of blood in the coronary arteries. Symptoms of a cardiac ischemic event may include chest pain and pain radiating through the extremities, but not all such events present these symptoms. Current medical intervention for severe acute ischemic events includes the administration of a class of drugs called thrombolytics that dissolve clots in the occluded coronary artery, and emergent PTCA, a medical procedure that opens the artery by inflating a balloon inside the clot to make a passage for circulation.
Traditionally, acute cardiac ischemic conditions are detected in a hospital setting by a physician's visual evaluation of 12-lead ECG waveforms. A physician typically makes an assessment by comparing selected features of a patient's ECG waveforms with equivalent features of other persons' ECG waveforms that are representative of various abnormal conditions. A physician may also look at the patient's ECG waveforms over time and evaluate any changes in waveform shape. A number of waveform features have been identified as useful in diagnosing acute cardiac ischemic conditions. Customarily, a physician observes the extent to which the "ST" portion of a waveform exceeds the isopotential line (i.e., the ST elevation) and uses this information to determine if an acute ischemic event has occurred.
The amount of damage done to a heart by an ischemic event depends, in part, on the amount of time that lapses before treatment is provided. Therefore, ECG data should be evaluated as early as possible so that functional changes associated with cardiac ischemia can be detected and reported as early as possible. With early detection of acute cardiac ischemia, appropriate treatment can take place as early as possible and, thereby, maximize the preservation of myocardium. The American Heart Association recommends that a patient with suspected acute cardiac ischemia be evaluated by a physician using a 12-lead ECG within ten minutes of arrival at a hospital emergency department. Unfortunately, outside of a hospital, highly trained medical personnel are not always available to meet a patient's immediate needs. Quite often, a first responding caregiver to a patient in the field may be a lesser trained individual that is not competent in evaluating ECG waveforms to detect acute cardiac ischemic events.
Furthermore, it can be challenging for lesser trained individuals outside of a hospital to use a conventional 12-lead ECG system that requires proper placement of numerous electrodes on a patient's body. As noted earlier, a 12-lead ECG system requires ten electrodes to be spread across the chest and limbs of a patient. Serious misinterpretations may occur if the electrodes are incorrectly placed or connected. Lesser trained individuals may also have cultural inhibitions arising from placing electrodes at the required locations on the patient's chest. As a consequence, lesser trained individuals often delay obtaining ECG data for evaluating acute cardiac ischemic conditions until trained medical personnel are available. Each moment of delay, however, may seriously jeopardize the patient and result in further damage to the patient's cardiac tissue. A need, therefore, exists for a device that not only obtains ECG data in a simplified manner, but also quickly evaluates the ECG data and automatically produces a preliminary report of whether an acute cardiac ischemic condition has been detected.
While attempts have been made in some applications to reduce the number of electrodes required to obtain ECG data, these applications are generally directed to basic monitoring of a patient's heart rhythm for long-term patient observation and not for diagnosis of acute cardiac ischemic events. Devices for basic patient monitoring generally use a frequency response of about 0.5 to 40 Hz when at least three electrodes are connected. Devices for cardiac diagnosis generally have a wider frequency response of 0.05 to 150 Hz to maintain the full fidelity of the ECG without appreciable distortion. Furthermore, conventional ECG evaluation for diagnosing acute cardiac ischemia is more complex than basic patient monitoring; thus, the use of fewer than ten electrodes is discouraged, as such configurations provide a physician with fewer waveforms for analysis. The prior art has attempted to address this concern by using limited ECG data to approximate and extrapolate 12-lead ECG waveforms. Nevertheless, conventional ECG systems using fewer than ten electrodes are still perceived as providing a physician with an inadequate amount of data to accurately diagnose acute cardiac ischemia.
In recent years, there have been efforts to develop enhanced ECG waveform interpretation based on computer analysis. Conventional processes used for analyzing 12-lead ECG waveforms are typically based on heuristics derived from the experience of expert physicians. Such processes implement rules that attempt to simulate an expert physician's reasoning and therefore perform no better than the expert. In practice, many such processes often perform more poorly than human expert evaluation.
Furthermore, when a conventional heuristic process is used, it is often difficult to choose an optimal operating point for the device in terms of sensitivity (i.e., detecting true positives) and specificity (i.e., avoiding false positives). Generally speaking, a device tuned to be more sensitive is typically less specific, while a device tuned to be more specific is typically less sensitive. A typical sensitivity/specificity tradeoff is illustrated by a receiver operating characteristics (ROC) curve, an example of which is shown in FIG. 21. Using a conventional heuristic classifier, it is difficult to make the sensitivity/specificity tradeoff explicit; thus, the selection of an operating point on the ROC curve is often made in a suboptimal, ad hoc manner. As such, there is a need for a device and method that can provide better performance in terms of sensitivity and specificity and further provide for selecting a sensitivity/specificity tradeoff in a more systematic way.
Still further, a device and method are needed to decentralize the acquisition and evaluation of ECG data for acute cardiac ischemic conditions and provide a simpler and more accurate means for ECG evaluation that can be used by lesser trained individuals outside of a hospital. By providing analysis of ECG data further "out" from a hospital, earlier detection of acute cardiac ischemic conditions is anticipated, thus increasing the likelihood of survival of patients experiencing acute cardiac ischemic events. A device and method that use a reduced number of leads for automatic evaluation of ECG data for acute ischemic conditions are not known in the art. The present invention addresses these needs as well as other shortcomings in the prior art.